create ssh_agent_env with current user id in file name to avoid collisions with other users
create ssh_agent_sock with current user id in file name to avoid collisions with other users
If `SSH_AGENT_PID` is unset, grep will succeed if another ssh-agent is
running as it will match the string `ssh-agent` where the PID was an
empty string. Set a default value for grep to a value that will never
match if unset, i.e. -1.
Signed-off-by: Sorin Ionescu <sorin.ionescu@gmail.com>
